1. Field of the Invention
This invention relates to a conveying system for carrying works by means of a series of conveyers, and more particularly to a work conveying system including a lift, which is located between overhead conveyers and a ground-based conveyers so as to receive works from the overhead conveyers and to deliver them to the ground-based conveyers without interfering with the latter conveyers, thereby reliably and intermittently carrying the works from one processing stations to other processing stations in an automobile assembly line.
2. Description of the Related Art
A variety of processing devices are installed in an automobile assembly line having a limited available space where various works such as body panels are assembled to form an automobile. In such a case, the works are usually conveyed by conveyers such as overhead and ground-based conveyers according to body assembling procedures.
Lifts are usually used to bring the works between the conveyers which are installed at different heights in the automobile body assembly line. In this case, the conveyers are required to operate harmoniously with a timed relationship. The conveyers are controlled by their control units in response to status indicating signals from their cooperating conveyers. For example, work carrying members of two adjacent conveyers are controlled not to reach a work delivery area simultaneously. For this purpose, an interlock device is incorporated in the control units so as to prevent interference between the work carrying members of the two conveyers at the work delivery area.
FIG. 6 of the accompanying drawings shows an example of the foregoing work conveying system. With this system an overhead conveyer is installed on a ceiling of a factory building. A work 3 carried along a route U (not shown) is delivered to a ground-based shuttle conveyer 2 via a lift 1.
The lift 1 is includes a support 102 one end of which is fixedly secured on a floor of the factory building and the other end of which stands upright, a movable base 4 which reciprocates between an upper first delivery position P1 and a lower second delivery position P2, and a lift driver 101 which is positioned atop the support 102 and is connected to the movable base 4 so as to reciprocate the movable base 4 vertically. The movable base 4 has a retractile fork 4a for receiving the work 3 which reaches the first delivery P1 along the route U. The fork 4a projects horizontally from the movable base 4 so as carry the work thereon. On the contrary, when the fork 4a is idle, it is retracted into the movable base 4 so as not to interfere with the shuttle conveyer 2. The lift 1 is controlled by a lift controller which is electrically connected to the lift driver 101.
The shuttle conveyer 2 is installed on the floor at a position under the overhead conveying route U. The shuttle conveyer 2 carries works 3 on its shuttle member 6 to work stations.
The shuttle member 6 is moved up by a drive mechanism (not shown), receives the work 3 placed on a jig 5, and carries the work 3 to the succeeding processing station while it remains upward. At the destination, the shuttle member 6 goes down, and places the work 3 on another jig 5. Then, the shuttle member 6 keeps its lower posture, returns to its original position on a standby level P4, and stops there. The shuttle member 6 repeats the foregoing operation. The shuttle member 6 is driven by a shuttle driver 201, of which operation is controlled by a shuttle controller 8 according to the operation cycle of respective work stations. The operation cycle is determined based on the time required at respective work stations. The jigs 5 the at respective work stations are located outside a moving path of the shuttle member 6 so as not to interfere with the shuttle member 6.
Near the second delivery position P2, there is a zone Z where the moving path of the movable base 4 of the lift 1 overlaps the moving path of the shuttle member 6 of the shuttle conveyer 2. This zone Z is called the overlapping zone Z hereinafter. The work conveying system includes an interlocking device for preventing the interference between the movable base 4 and the shuttle member 6.
The interlocking device has two limit switches L1 and L2 attached on the support 102, for example. The limit switch L1 turns on when the movable base 4 is at the second delivery position P1, and sends a signal to the lift controller 7. The limit switch L2 is turned on or off when the movable base 4 passes through a waiting position P5 where the movable base 4 does not interfere with the shuttle member 6. The waiting position P5 is at the center of the moving path of the movable base 4. Specifically, the limit switch L2 is located near the waiting position P5, and is turned on when the movable base 4 moves up from the second delivery position P2 and passes the waiting position P5. On the other hand, the limit switch L2 is turned off when the movable base 4 moves down from the first delivery position P1 and passes the waiting position P5. In other words, the limit switch L2 remains on while the movable base 4 stays above the waiting position P5, so that a signal is provided to the lift controller 7 and a shuttle controller 8 (to be described later) to indicate the position of the movable base 4. When the movable base 4 is below the waiting position P5, the limit switch L2 is turned off to send a signal to the lift controller 7.
Furthermore, the shuttle conveyer 2 includes limit switches L3 and L4 for detecting the position of the shuttle member 6 in the overlapping zone Z, and the presence or absence of the work in the overlapping zone Z. The switches L3 and L4 are positioned near the jig 5.
The limit switch 3 is turned on when the work 3 is placed on the jig 5 at the second delivery position P2, and sends a signal to the shuttle controller 8 and the lift controller 7 so as to indicate the situation. The limit switch L4 is turned on by the shuttle member 6 which is on the standby level P4, and is turned off when the shuttle member 6 leaves the standby level P4, so that the switch L4 sends a signal to the shuttle controller 8 and the lift controller 7 to indicate the situation.
When the movable base 4 receives a work 3 at the first delivery position P1, the lift controller 7 descends the movable base 4 so long as a preceding work 3 is not at the second delivery position P2 according to the limit switch L3, and the shuttle member 6 is on the standby level P4 according to the limit switch L4. The movable base 4 is moved down to bring the work 3 to the second delivery position P2 via the waiting position P5. On the contrary, the lift controller 7 controls to stop the movable base 4 at the waiting position P5, when the lift controller 7 receives signals from the limit switches L3 and L4 indicating that the work 3 is at the second delivery position P2 or that the shuttle member 6 has is not on the standby level P4, respectively.
On receiving a signal from the limit switch L2 indicating that the movable base 4 is above the waiting position P5, the shuttle controller 8 ascends the shuttle member 6 from the standby level P4 to the second delivery position P2, where the shuttle member 6 receives the work 3. Then, the shuttle member 6 is moved to a third delivery position P3.
When receiving a signal from the limit switch L2 indicating that the movable base 4 has reached the waiting position P5, the shuttle controller 8 moves the movable base 4 down to the second delivery position P2, where the work 3 is placed on the jig 5. Then, the movable base 4 is ascended with the fork 4a retracted. Then, the shuttle member 6 is controlled not to be actuated until the movable base 4 is moved up above the waiting position P5.
The foregoing operation keeps the lift 1 and the shuttle conveyer 2 from interfering with each other in the overlapping zone Z.
The operation of the shuttle conveyer 2 is controlled according to the operation cycles of the processing machines on the assembly line. In other words, the start time and cycle time of the shuttle conveyer 2 depend upon the machines which process the works 3. Therefore, even when there is no work 3 at the second delivery position P2, the shuttle conveyer 2 has to move up from the standby level P4 and start the cycle to convey works 3 to succeeding processes. Therefore, the shuttle conveyer 2 is sometimes idle at the second delivery position P2. The following inconveniences will be caused under such a condition.
Assume that the limit switch L3 signals that the work 3 is at the second delivery position P2, and that the limit switch L2 signals that the movable base 4 is above the waiting position P5. Under this condition, no trouble will be caused when the shuttle member 6 begins to ascend from the standby level P4.
On the contrary, assume that when the shuttle conveyer 2 is driven even if no work 3 is present at the second delivery position P2, i.e. when the limit switch L3 signals that no work 3 is at the second delivery position P2 and when the limit switch L2 signals that the movable base 4 is above the waiting position P5. And assume that the shuttle conveyer 2 is actuated under this condition according to its operating cycle. When the movable base 4 moves downwardly from the first delivery position P1 and is at just before the waiting position P5, it receives the signal indicating that no work 3 is at the second delivery position P2. Under this condition, since the limit switch L4 signals that the shuttle member 6 is on the standby level P4 at the moment or immediately after the shuttle member 6 starts its upward movement from the standby level P4, the interlocking device will not function, so that the movable base 4 keeps on moving down via the waiting position P5.
Then, the movable base 4 is stopped by the interlocking device above the waiting position P5 in response to the signal from the limit switch L4 indicating that the shuttle member 6 is not on the standby level P4.
On the other hand, the moving shuttle member 6 is also stopped in response to the signal from the limit switch L2 indicating that the movable base 4 moves down to a position below the waiting position P5.
The conventional conveying system suffers from the foregoing inconvenience that the lift 1 and the shuttle conveyer 2 are unintentionally stopped by the interlocking device according to their operation timings, so that the work 3 cannot be delivered as desired.